From DE 10 2008 003 272 A1 it is known to survey a photovoltaic system with regard to its function. For this purpose, a motion sensor is arranged at the solar modules. This motion sensor allows for detecting movements in case of theft or due to strong wind or storm. Further, a temperature sensor is provided which surveys the environmental temperature increasing in case of a fire. By means of surveying the power produced by the solar modules it shall also be recognized whether snow lies on the photovoltaic system or whether any contamination affects the performance of the system.
A method of surveying solar driven power plants is known from DE 10 2006 008 178 A1, in which the produced power is continuously monitored by comparing the power measured for its individual solar modules. Such comparisons may also be made between a fixed reference module and at least some of the solar modules. Preferably, the reference module is a module which has the same physical properties as the solar modules and which is exposed to the solar light intensity in direct proximity to and under the same conditions as the solar modules. In DE 10 2006 008 178 A1 a contamination of and damage to the solar modules by weather conditions are indicated as variables that have an influence on the power produced by the solar modules.
An array of photo sensors which is arranged with respect to a shadowing device is known from US 2009/0314279 A1. The signals of the individual photo sensors are used to orient a solar module which is in a fixed arrangement with the array and the shadowing device in an optimum way with regard to the position of the sun.
A uniform contamination both of an insolation sensor and the associated solar modules may not be detected by comparing the solar light intensity detected with the insolation sensor with the electric power produced by the solar modules. Similarly, all other external influences which affect both the insolation sensor and the solar modules may not be resolved by means of a comparison between them.
Thus, there still is a need for an insolation sensor by which external influences, particularly due to precipitation, can be detected even if these external influences both affect the insolation sensor and the solar modules surveyed by means of the insolation sensor.